Type-graphql: Support entity inheritance

[artman]

In order to maintain a large number of entities it would be beneficial to be able to use inheritance to not have to redefine entity fields.

The following demonstrates this:

class BaseEntity {
  @Field({ description: "The identifier of the entity." })
  public id: string;

  @Field({ description: "The time at which the entity was created." })
  public createdAt: Date;

  @Field({ description: "The time at which the entity was updated." })
  public updatedAt: Date;
}

@ObjectType({ description: "A user" })
export User extends Model {
  @Field({ description: "The name of the user." })
  public name: string = "";
}

[artman]

My bad. Decorating the base class with ObjectType will make this work.